Coniferous seedlings, for example, are raised from the seed of conifers, for example, in nurseries, in a greenhouse and/or an open land outdoors. Indoor cultivation normally involves the use of a very large number of small containers or pots that contain a growth substrate, for instance peat, in which one or more seeds are initially implemented. One pot is normally used for raising one (tree) seedling. When the seedling has reached a certain size, it is normally moved to an outdoor location for a certain amount of time, so as to acclimatise the seedling approximately to the climate that will prevail at the future growth site of the seedling, or sapling, subsequent to being planted in a forest area, for instance. Normally, a large number of pots (tens-hundreds) are joined together to form a cassette.
A large number of cassettes are placed in, e.g., rectangular forcing benches or frames that have four low walls and a generally open bottom, for instance comprising a number of mutually spaced bars or struts. These forcing benches or frames rest, e.g., on the short sides of a support with the bottoms of the benches or frames spaced, for instance, from ten to fifteen centimeters above the greenhouse floor, meaning that air will circulate around all pots, including the bottoms of the pots. It is endeavoured to cover the largest possible area of the floor surface in the greenhouse with these forcing benches or frames. These forcing benches or frames are movable and can be shifted from the greenhouse to an ambient outdoor area with the aid of a truck for instance. It is important to note that greenhouses are typically placed directly on the ground. Normally, nurseries, for example, are erected in places where the ground naturally consists of sand and/or gravel. Such grounds are often relatively flat by nature, and any litter present can be easily removed, and the ground smoothed out when necessary to form a generally even and horizontal greenhouse floor.
In order that the seed in each pot will produce a seedling and, above all, to ensure that the seedling will be given the chance to grow to a size in which it can be planted outside, for instance in forest land, it is necessary to add externally fertilizer to the growth substrate in the pot, for instance the peat, at given intervals. It has long been usual to use a certain nutrient mixture that contains both macro nutrients and micro nutrients. Necessary macro nutrients are nitrogen (N), calcium (Ca), phosphor (P), sulphur (S), potassium (K) and magnesium (Mg). Necessary micro nutrients are iron (Fe), manganese (Mn), copper (Cu) molybdenum (Mo), boron (B), zinc (Zn), chlorine (Cl) and nickel (Ni). Calcium is normally added separately to the growth substrate in the form of lime. There are chemical suppliers who in an industrial way produce fertilizers containing the other above mentioned nutrients. These fertilizers are not produced by mixing together the elements as such, but by mixing together a number of chemicals, normally salts, in which the elements are present. As the name implies, the macro nutrients are present in the mixture in a much higher proportion than the micro nutrients. Furthermore, the quantities between respective macro nutrients and between respective micro nutrients also differ, as does also the percentages. The aforesaid nutrient mixture or fertilizer can be purchased commercially in the form of a relatively concentrated aqueous solution.
This aqueous solution is purchased by nurseries and diluted with water to a suitable concentration of the chemicals concerned in aqueous solution, prior to use. Greenhouses are normally equipped with a sprinkler system through which the nutrient mixture in question is delivered to each clump of growth substrate in each pot. The sprinkler system includes, for example, a transversal pipe whose length is slightly greater than the combined width of the forcing benches or frames in a lateral row. One end of the pipe is connected to a flexible hose whose length corresponds generally to the length of the greenhouse. The opposite end of the pipe is plugged and nozzles are disposed at regular intervals along the pipe. The pipe and the hose attached thereto extend in two rails which extend longitudinally along the greenhouse and which are mounted in the roof structure of the greenhouse for instance. The arrangement concerned, including the pipe and the hose, is driven mechanically from one short side of the greenhouse to the other short side thereof, and back again. The hose connects the pipe to which it is fitted to a vessel that contains the nutrient mixture or the fertilizer in the form of said aqueous solution, which is pumped into the pipe and exits therefrom through the nozzles so as to be sprayed evenly over all plants/seeds in their individual clumps of growth substrate in respective pots, as the arrangement is advanced at a uniform speed over all forcing benches or frames and their respective cassettes. The described supply of nutrients to the plants, via the clumps of growth substrate takes place at most once a day, and is today often fully automated and fully computer controlled.
The macro nutrient nitrogen is of particular interest in the present context. According to the techniques known up to now, nitrogen in nitrate (NO3−) form and nitrogen in ammonium form (NH4+) has been used and traditionally, a mixture of these two nitrogen sources has been used. A concentration of 61.5% nitrate and 38.5% ammonium has long been considered a decisive blessing in respect to the optimum growth of conifer plants for instance. In the Swedish Patent Specification 323 255 (with patent of addition 331 610) Torsten Ingestad et al present a method of stimulating the growth of green plants by adding a mineral nutrient. The specialty with the method was (and is) that the mineral nutrient in a steady state shall be held in a form that can be taken up by the plants as a salt mixture, in which the element proportions correspond essentially to the proportions present in the plant at optimum growth. The elements present in the fertilizer are in accord with earlier calculated macro nutrients. According to the patent, a decisive feature is that the salt mixture includes a relatively large proportion of nitrogen in respect to other given macro nutrients, although to varying degrees. The fact that the nitrogen is present in a large quantity relative to phosphorus is of particular importance. The majority of the micro nutrients recited earlier are also included in the patent in question and are mentioned as trace substances. According to the exemplifying embodiments recited in the patent specification, both ammonium and nitrate are used as a nitrogen source, i.e. a mixture of those compounds. However, the question of a nitrogen source is not taken up explicitly and no mention is made of the distribution of precisely 61.5% nitrate and 38.5% ammonium. This determination must have been made in some other context or by some one else. Although the distribution of 38.6% and 61.4% is mentioned on page 9 of the Swedish Patent Specification, the distribution is not concerned with a distribution between nitrogen in ammonium form and nitrogen in a nitrate form, but with distribution of a trace substance mixture in accordance with what has been earlier described, plus a complex former EDTA, to the solution A and to the solution B in example 2.
Measurements carried out in nurseries concerned with raising conifer plants in which the aforesaid traditional nutrient mixture has been used as a fertilizer, have shown that only 30 to 40% of the nitrogen supplied is taken up by the seedlings (plants), whereas the remainder, i.e. 70-60%, goes to waste. The nitrogen disappears in several ways. A minor part of the nitrogen supplied never reaches its target, i.e. the clumps of growth substrate or root balls, but lands directly on the greenhouse floor, i.e. on the ground. One reason for this is that although strenuous efforts are made to ensure that the forcing benches or the frames are placed flush against one another, narrow cracks will always occur there between, and also as a result of the need to provide a number of narrow inspection corridors in the greenhouse. Moreover, because the aqueous solution containing the nutrients is sprayed or showered over the width of the greenhouse, or more accurately over the full width formed by a lateral row of forcing benches or frames, it is unavoidable that some of the aqueous solution (albeit only a small part) will land directly on the floor and directly enter the ground. A significant part of the nitrogen loss is due to some of the aqueous solution draining through the clump of growth substrate, for instance of peat, and out through the round holes or long strip slits present in the pots and therewith drop down onto the floor, i.e. the ground. The form in which the nitrogen is administered is significant in this respect. It has been found that the ammonium ion NH4+ is absorbed by the growth substrate to a much higher degree and remains in the substrate to a greater extent than in the case of the nitrate ion NO3− which has a high degree of mobility, wherewith the major part of the ion passes straight through the growth substrate without fastening thereto. (The same thing occurs when fertilizing with ammonium nitrogen and nitrate nitrogen in woodlands in order for the trees to benefit from the nitrogen). It is not known whether a small part of the nitrogen in the sprayed nutrient solution is converted to ammonia and leaves the supply site via the air. Since systems similar to those used to fertilise plants indoors are also used to raise plants outdoors, corresponding leakage problems to the ground soil also occur with outdoor planting. In this context, another drawback with outdoor planting is that leaching of primarily the nitrate nitrogen, but also the ammonium nitrogen, is aggravated by the sometimes occurring rain.
The above described spillage or leakage of nitrogen constitutes problems. One problem resides in that an unnecessarily large amount of fertilizer is applied, which is uneconomical. Another and more serious problem is that the leaking nitrogen contaminates the ground and, as earlier mentioned, the nitrate ion in particular has a large tendency to penetrate through soil masses and ultimately reach the ground water, which is most undesirable.
It is proposed in the literature that with regard to concentration, the ammonium ion shall be preferred to the nitrate ion in the fertilizer. According to certain experts, this preference shall go as far as excluding the nitrate ion completely so that the ammonium ion will become the sole nitrogen source for the plants. Several persons skilled in this art are conscious of the problem associated with the nitrate ion, as described above.
Also found in the literature are proposals regarding quantitative differences between different macro nutrients that deviate from the traditional view of the experts. Also found in the literature is a single proposal of using an organic nitrogen source either in or as a fertilizer.
Earlier proposals for the use of the ammonium ion as a nitrogen source, either substantially or completely, is a step in the right direction from the aspect of leakage, and therewith also from an environmental aspect, since the ammonium ion is relatively immobile in growth substrates and consequently the major part of the nitrogen applied in ammonium form will remain in the growth substrate and gradually be taken up by the plant.